1. Field of the Invention
The present invention relates to a waste heat boiler which is simple in structure and small in size so as to occupy only a small space, and which has an extended range of heat-to electricity ratios which allows it to be used in a wide range of applications. The boiler is effective to carry out three functions, i.e. heat recovery, full firing of exhaust gas, and a new (or auxiliary) burning, in connection with or independently of a heat-absorbing water tube furnace-inserted type combustion chamber (hereinafter referred to as a tube-nested combustion chamber), which was invented by one of the applicants for the present invention, applied to waste heat recovery equipment such as a cogeneration system (combined heat and power generation) or a combined cycle system (combined power generation). The invention further relates to a method of waste heat recovery using the aforementioned boiler.
2. Description of the Prior Art
Heretofore, in the so-called cogeneration systems or combined cycle systems which utilize heat engines, the use of a waste heat recovery boiler has always been necessary for recovering waste heat therefrom. There have been available two basic arrangements for installing the boiler into the system.
In a first such arrangement, an exhaust-gas heat recovery boiler is installed so as to attain the greatest possible heat recovery from the exhaust gas derived from the engine or gas turbine.
In this first arrangement, the recoverable waste heat quantity or the steam generation is limited, relative to the quantity of electric power to be generated (or output), by the so-called pinch point concept wherein the exhaust gas temperature cannot be lowered below the saturation temperature of the boiler. Accordingly, the ratio of the calories generated to the electric power produced, that is, the so-called heat-to-electricity ratio, is fixed.
On this account, there has been a problem that the utilization of this system is restricted by the fixed nature of the heat-to-electricity ratio because certain users, such as hotels, petroleum refineries and the like, require high heat-to-electricity ratios whereas other users, such as offices, cement industries and the like, require low heat-to-electricity ratios. Thus, in order to expand the range and objects of its application, it has been demanded that the range of the heat-to-electricity ratio be expanded in the cogeneration or combined cycle systems.
A second arrangement, on the other hand, is such that the heat portion of the heat-to-electricity ratio can be arbitrarily changed by charging and burning additional fuel and air (i.e. auxiliary burning) in an auxiliary chamber provided to the waste heat boiler of the first arrangement. This second arrangement is being successfully put into practical use.
In improving the heat-to-electricity ratio in the cogeneration system or the combined cycle system as described above, the systems have resorted, in large part, to upgrading the efficiency of an engine or an electric power generator in generating electricity. As this efficiency increases, the heat portion of the ratio decreases relative thereto, and causes the heat-to-electricity ratio to diminish down to a limited constant value.
To remedy this situation, it has been proposed to increase the heat portion. A method which has been commonly employed for this purpose utilizes a duct burner (CD), as shown in the schematic-diagram of FIG. 2, installed at the entrance of the aforementioned heat recover boiler B, and a necessary amount of fuel 7 is charged into the duct burner CD and then burned by exhaust gas.
In this instance, the exhaust gas from an ordinary gas turbine contains about 15-16% oxygen. By charging the necessary amount of fuel 7 into the duct burner CD, the calorie production can be increased 1-5 fold relative to that of the exhaust gas recovery alone (i.e. without utilizing the duct burner).
The duct burner of this type needs to be installed between the exit of a heat engine (e.g. a gas turbine (GT)) and the waste heat boiler (B). This burner is typically large and since the O.sub.2 % in the exhaust gas is lower than that in the ambient air, its flame is considerably elongated, but according to a traditional regulation, there is a requirement that this flame should not reach the heating surface of the waste heat boiler (B). Thus, a substantially larger space has been required by the addition of the duct burner (CD).
Among the requirements to meet the demand for expanding the range of the heat-to-electricity ratio, steam generation is often required even when there is no power demand. Actually, it is an essential requirement of the waste heat boiler of this type that the three types of functions are provided, including generation of steam by use of only ordinary fuel, as well as the recovery of exhaust-gas as mentioned above with respect to the first type of arrangement and full firing of the exhaust gas as mentioned above with respect to the second type of arrangement.
However, with the conventional waste heat boiler of this type, it was impossible to fully meet the aforementioned requirements.
More specifically, in the prior art exhaust-gas fully fired type boiler B, it has been necessary to provide a duct burner as a combustion chamber for burning the fuel in order to provide the functional requirements mentioned above, as shown in the schematic diagram of FIG. 2. Thus, the provision of a space for installation of the combustion chamber (i.e. duct burner) was always necessary. It has also been necessary in the prior art, in order to allow for the generation of steam when the heat engine (e.g. gas turbine) is not in operation or is only slightly operating and thus producing little or no exhaust gases, to provide the boiler B with an auxiliary combustion chamber 3 (see FIG. 4B) and an auxiliary burner device 4 for providing auxiliary burning of auxiliary fuel 41 and air 42. The need to provide this auxiliary chamber 3 has also added to the space requirements of the system.
FIG. 2 is a schematic diagram of a conventional gas turbine cogeneration system, showing a compressor CP into which Air A is supplied, a combustor CB into which Fuel F is supplied, a gas turbine GT, the duct burner CD, a duct D, the boiler B and a denitrator DNO.sub.x. The boiler B shown in FIG. 2 is, for example, a water-tube type waste heat boiler (B), as shown in FIG. 3, which is used as a simple waste heat boiler.
FIG. 3 shows a boiler B which includes therein water cooled wall tubes 1 and water tubes 2.
The conventional gas turbine cogeneration system of FIG. 2 is so designed that the fuel F charged into the duct burner CD may be completely burnt within the duct D. Accordingly, the waste heat boiler B may be of the conventional simple boiler structure. However, when no exhaust gas exists (e.g. when the gas turbine GT is not operating) and it is desired to generate steam to improve the heat-to-electricity ratio, for example, with the conventional system of FIG. 2, an auxiliary combustion chamber, burners, and auxiliary fuel for combustion, and a device for supplying combustion air are separately required since normally the duct burner shown in FIG. 2 is not provided with a device for feeding air for combustion. Consequently, the boiler B must be provided with the auxiliary combustion chamber 3 and will thus have the shape shown in FIG. 4.
Thus, FIG. 4 shows an example of the prior art waste heat boiler which includes both a duct burner CD (as in the FIG. 2 prior art) and an auxiliary combustion chamber 3. Combustion chamber 3 includes an auxiliary burner 4 for burning auxiliary fuel 41. The auxiliary fuel 41, the fuel 7 for the duct burner (CD), and combustion air 42 are fed and burned in the combustion chamber 3, after which they enter the tube chamber having the heat transfer tubes 2 and are exhausted from an exhaust gas outlet 6.
Exhaust gas derived from a heat engine, such as the gas turbine GT, is introduced into the duct burner CD as indicated at 7. The fuel 7 is injected by a burner of the duct burner into the exhaust gas to burn in the form of a flame in the duct burner. To accomplish this, according to the conventional technical concept, a combustion chamber within the duct burner has also required a large space, as is the case with the previously discussed combustion chamber 3 of the boiler, because the flame was prohibited from making contact with the water tubes 1 or the heat transfer tubes 2 of the waste heat boiler B, in order to avoid the possibilities of quenching of the flame and overheating of the water tubes.